Method and apparatus for manufacturing precision lenses



April 12, 1960 E. E. FLUSKEY ETAL' 2,932,138

METHOD AND APPARATUS FOR MANUFACTURING PRECISION LENSES Filed Nov. 10,1958 F IG.3J8. 3 %I It L FIG. 2. 2O

26 x 16 l -\i 24 Z2 30 an 12 FIG. 5. 5a

56 INVENTORS $331 31? LENS EDWARD E. FLU SKEY HUBERT U. BRUECKNER BY fa/ATTORNEY METHOD AND APPARATUS FOR MANUFACTUR- llNG PRECISION LENSESApplication November 10, 1953, Serial No. 772,942

1 Claim. (Cl. 51-209) v This invention relates to an improved means forgrinding and polishing lenses to the extremely close tolerances requiredin optics and in the manufacture of precision instruments. Moreparticularly it relates to novel lapping devices provided with a meansfor altering the curvature of the lapping surface to compensate for theunequal wear which is characteristic of known lapping devices and torestore the lapping surface to the precise size and configurationrequired for the production of the desired lens surface curvature.

In presently practiced methods of grinding and polishing lenses, one ormore lenses is secured in a blocker by means of pitch or other adhesivematerial, as described for example in United States Patents 689,933 and2,582,087.

When convex lenses are being ground or polished the single lens orblocker is mechanically rotated under a grinding or polishing lap whichhas been made to approximately the same radius of curvature as thatdesired on the finished lens or lenses. The lap is charged with anabrasive such as Carborundum, emery, precious stone dust or rouge,suitable for the type of glass employed, while the lap under pressure,is oscillated over the spherical surface of the lens or lenses beingeither ground or polished.

When concave lenses are being ground or polished, the process isidentical to the method described above, with the exception that thelenses are placed over and above a suitably curved lap under pressureand oscillated over the surface of the lap, which is rotating at asuitable speed.

In the grinding or polishing operation the surface of the lens isaltered in the desired fashion by the action of the abrasive. At thesame time the surface of the lapping tool, even when made of suchdurable material as the cast iron known as Meehanite, is found toexperience an undesirable change in shape due to the uneven wearinherent in the process. In the grinding or polishing of convex lenssurfaces, wear of the lapping tool is greatest along the periphery ofthe tool face and least at the center of the lapping head. In shaping oflenses of concave shape the wear experienced is also greatest along theperiphery of the lapping surface and in both instances this is inherentin the process and does not depend on the specific shapes or specificmaterials involved.

In precision optics, lenses are ground to Newtonian bands (1 light bandis approximately 0.0000116 inch), and hence the accuracy of the contourof the lapping surface is of primary importance.

As grinding and polishing of lenses is presently practiced, skilledcraftsmen known in the optical trade as lens grinders or lens polisherswho from long years of training and experience have acquired an abilityto correct or compensate for lap or tool wear must intermittentlycorrect the lapping surface to maintain the precise accuracy of radiusof curvature required in the finished lens.

States Patfi Ice] Since configuration of the lap is produced by preciseand expensive processes it is necessary that the laps must be used tothe fullest possible extent in order to permit the required economy ofoperation for competitive operation. Therefore, in practice, it isusually necessary to cut down any high spots which develop in use so asto restore the surface of the lap to the desired curvature, so that thelap may be reused.

The usual method of lap or tool correction is for the skilled technicianto lap the surface requiring correction, by holding the same over arotating cast iron block of a similar curvature and to laboriously lapwith a suitable abrasive until the correction of curvature is obtained.Another method is to rotate the lap requiring correction, and to use ahand tool to cut away at the radius until the desired curvature isobtained. Since these corrections are pursued on a trial and errorbasis, by the skilled technician, they are very time consuming andrequire his absence from the lens grinding or polishing machine with aconsequent loss of productive output.

In order to minimize this serious and repetitive production loss, wehave invented a lap which has incorporated into the same, a means bywhich the lens grinder can make the minuscule corrections requiredwithout suffering the dual production losses entailed by his absencefrom his production machines and the laborious hand cutting or lappingpreviously required under prior art arrangements.

One object of the present invention is to provide a means whereby thecontour of the lap may be restored to the precise curvature desiredwithout recourse to the laborious cutting operations heretoforepracticed.

Another object of the present invention is to provide a means wherebythe extent to which the curvature of the lap is altered may be readilycontrolled by the lens grinder.

Still another object of the present invention is to provide a meanswhereby the curvature of the lapping surface may be either increased ordecreased an amount directly related to a scale carried on the means foradjustment.

These and other objects will become more apparent from the followingdescription and drawings in which:

Figure 1 is a view of the concave lap of Figure 2 as seen from the top;

Figure 2 is a front elevation, shown partly in section taken along line22 of Figure 1;

Figure 3 is a view of the lap of Figure 2 as seen from the bottom;

Figures 4, 5 and 6 are similar views for a convex tool or lap, and

two flat portions 13 to facilitate the adjustment of the curvature ofthe lap as later described.

Body portion 14 is defined by a working face 16 possessing the curvatureto be imparted to the lens or lenses to be ground and extending from acrown 18 at its center to a rim 20 at its periphery and by an oppositeface 22 which comprises an arcuate portion extending from shank 12 to acircle of a maximum diameter 24 and terminating in a threaded peripheralband 26 between rim 20 and circle 24.

I tion.

A plurality ofslits 30 extend radially inwardly from the threaded bandto a point on the inner working surface intermediate of the crown 18 andthe peripheral rim 20. Slits 30 are cut through the body portion 14 soas to extend from the working surface 16 to the opposite surface 22 ofthe body. In the embodiment shown, these slots were formed by drillingeight holes in the lap distributed symmetrically about a central axisand then sawing the slits 30 outwardly to the threaded periphery. Agreater or lesser number of slits may be employed depending on theamount of flexibility it is desired to add to the natural resilience ofthe cast iron tool.

A compensating ring 32 having an annular shape is threaded onto thethreaded portion of the lap body. One surface of the ring 32, carries ascale 34, which is adapted to cooperate with an indexing means 36 on thelap body. An arrow is shown in Figure 1 for this purpose. For even fineradjustment, a Vernier could be substituted for the arrow 36. Otherindexing means may be employed or the scale and pointer may beinterchanged, if desired without departing from the intended scope ofthe inven- Two flats 38 are provided to receive a wrench or spanner,when adjusting the lap.

In Figures 4-6 corresponding portions of the convex lap shown have beengiven identifying primed numerals,

corresponding to the unprimed numerals for Figures l-3.

In Figure 7 there is shown in greatly exaggerated form and for purposesof illustration, the progressive wear experienced by a lap in actualservice. As noted above the wear inherently increases progressively froma minimum at the crown to a maximum along the periphery of the lap.

Our novel lap is manufactured by casting a suitable shape and finishingit to the desired contour in the usual manner. Thereafter a thread isout along the rim, and radial slits are sawed in the lap body as shown.The device is assembled after complementary threads have been cut in thering 32.

By means of the structure described above it is possible for the lensgrinder to alter the radius of curvature of the working face of the lapby minute amounts in a precise and determinable manner and the followingexplanation illustrates one specific combination found suitable forachieving the desired result.

One typical lap formed in accordance with our invention had thefollowing dimensions:

Radius of curvature-0.8950 Number of slots8 Threads-40 threads per inchAngle of thread-1 The major outside diameter of lap 10, ringportion 26,was threaded40 threads perone inch being selected only for purposes ofillustration. The thread was cut on a one degree taper in relation toits horizontal axis. Compensating ring 32' was also provided with asimilar pitch thread, at the same common angle. To adjust the lap forsize, it was placed in a fixture adapted to receive shank 12 and the twoflat portions thereof, which prevented rotational movement of the shankand body portions.

The compensating ring was then rotated by means of a wrench or spannerlaid across flats 38, to either diminish or increase the radius ofcurvature, according to the direction of rotation.

Arrow 36 and scale 34 permit the operator to gage the amount ofexpansion or contraction of the lap.

It will be readily appreciated that the amount ofv di-. ametralcontraction or expansion is one-half of the prod net of the pitch(expressed in inches) multiplied by the tangent of the angleof taper.

With a linear pitch of 0.025 inch with an angle of 1' (taper), theproduct of the two factors is 001745 X0025 or 0.00043625 inch. Thediametral movement is twice this or 0.0008725 inch.

The amount of change in diameter of the lap for each graduation on scale34, depends on the number of gradnations. If one hundred and twentygraduations are provided, each corresponds to 3 degrees of are. With arotation of 3, the diameter. of the lap is changed 0.0008725" divided byor 0.00000727 inch which is a little more than half of a Newtonian band.

As a practical matter, the choice of pitch and angle is dictated by thesize of the lap or tool and it should be evident that other combinationsof thread and taper could be used without departing from the scope ofthe present invention.

While we have described a preferred form of device for preciselyadjusting the contour of a resilient lap produced in accordance with ourinvention, it will be apparent that other means such as expandingWedges, eccentrics, cams and other diverse forms could be employed toachieve the desired result and such means are intended to becomprehended as within the scope of our invention.

We are aware of prior art efiorts to adjust the radius of curvature of alap by means located along the central axis of the lap, but suchadjustments are not uniform for a uniform amount of rotation and are notadapted to the'precision obtainable with the circumferential adjustingmeans constituting the present invention.

Having now described our invention in accordance with the patentstatutes, we claim:

An adjustable lap for manufacturing precision lenses which may beadjusted to compensate for wear without the necessity of lapping orremachining to restore said lap to its original size or magnitude ofcurvature, said lap consisting of a shank portion extending between aterminal portion adapted to be received in a chuck on a drivingmechanism and joined at its other extremity to a generally cup-shapedbody portion integral therewith comprising a working face and anotherface, the working face extending arcuately from the central axis of thelap to. a circular rim constituting the lip portion of said cup; theother face having an arcuate configuration corresponding generally tothe configuration of the working face and extending from the junction ofthe shank portion and the cup-shaped body portion toward the lip portionof said body; a plurality of slots extending through the body portion ofsaid lap radially disposed about the central axis of said lap; timepitch tapered threads bridging the surface of said lap between the lipof the body portion and the maximum diameter portion of the othersurface of said lap; a compensating ring bearing fine pitch threadsmating with the said tapered threads on said lap; a scale and indexmeans located each on one of said compensating ring and said lap body,where by the amount of relative movement between said body and said ringmay be precisely controlled and the adjustment in the curvature of saidworking face may be precisely effected in terms of said graduations tocompensate for wear.

References Cited in the file of this patent France May 12,

